I propose to study the cellular nature of the age-dependent delay in recovery of immune competence of C57B1/6 mice treated with low dose ionizing radiation by determining whether the delay recovery of immune competence in irradiated old mice is due to temporary inability of stem cells to differentiate into immunoccompetent cells, or to production of immunoincompetent cells. In addition, I plan to determine whether recovery of hematopoiesis is also delayed. Both in vivo and in vitro assays will be used to enumerate the number of stem cells and their T and B cell progeny. Specific assays for T cell activity (response to mitogens, delayed skin hypersensitivity, suppressor and helper functions, etc.) and B cell activity (response to mitogens, Ig-secreting capacity antibody production, etc.) will be used as tests of function. I expect the results to shed light on how age affects the ability of stem cells to undergo self renewal and differentiation during periods of regeneration.